In a vehicle such as a hybrid car, which includes an engine and an electric motor as driving sources, and an electric vehicle, a battery mounted on a vehicle body is charged, and driving power is generated with use of electric energy supplied from the battery. In general, a power supply circuit related to the battery is configured as a high-voltage circuit using as high voltage as 200V or higher, and for ensuring safety, the high-voltage circuit including the battery is in an ungrounded configuration in which the high-voltage circuit is electrically insulated from a vehicle body serving as a reference potential point of a ground.
In the vehicle mounted with the ungrounded high-voltage battery, a ground fault detection apparatus is provided to monitor an insulating (ground fault) state between the vehicle body and a system provided with the high-voltage battery, specifically, a main power supply system ranging from the high-voltage battery to a motor. As the ground fault detection apparatus, one of a type using a capacitor called a flying capacitor is widely used.
FIG. 11 illustrates a circuit example of a conventional ground fault detection apparatus of the flying capacitor type. As illustrated in FIG. 7, a ground fault detection apparatus 400 is connected to an ungrounded high-voltage battery 300 to detect a ground fault of a system provided with the high-voltage battery 300. In this apparatus, insulation resistance between a positive-electrode side of the high-voltage battery 300 and a ground is referred to as RLp, and insulation resistance between a negative-electrode side and the ground is referred to as RLn.
As illustrated in FIG. 7, the ground fault detection apparatus 400 includes a detection capacitor C1 operated as the flying capacitor. The ground fault detection apparatus 400 also includes four switches S1 to S4 around the detection capacitor C1 to switch a measurement path and control charge/discharge of the detection capacitor C1. The ground fault detection apparatus 400 further includes a switch Ss configured to sample voltage for measurement corresponding to charge voltage of the detection capacitor C1.
To figure out the insulation resistance RLp and RLn, the ground fault detection apparatus 400 repeats a measurement operation with one cycle including V0 measurement period→Vc1n measurement period→V0 measurement period→Vc1p measurement period. In any of the measurement periods, the detection capacitor C1 is charged with voltage to be measured, and charge voltage of the detection capacitor C1 is then measured. The detection capacitor C1 is then discharged for the subsequent measurement.
In the V0 measurement period, voltage corresponding to voltage of the high-voltage battery 300 is measured. Thus, the switches S1 and S2 are turned on, the switches S3 and S4 are turned off, and the detection capacitor C1 is charged. That is, the high-voltage battery 300, a resistor R1, the detection capacitor C1, and a resistor R2 constitute the measurement path.
At the time of measurement of charge voltage of the detection capacitor C1, the switches S1 and S2 are turned off, the switches S3 and S4 are turned on, the switch Ss is turned on, and sampling is performed in a control unit 420. Thereafter, the detection capacitor C1 is discharged for the subsequent measurement. Operations at the time of measurement of charge voltage of the detection capacitor C1 and at the time of discharge of the detection capacitor C1 are similar in the other measurement periods.
In the Vc1n measurement period, voltage on which an influence of the insulation resistance RLn is reflected is measured. Thus, the switches S1 and S4 are turned on, the switches S2 and S3 are turned off, and the detection capacitor C1 is charged. That is, the high-voltage battery 300, the resistor R1, the detection capacitor C1, a resistor R4, a ground, and the insulation resistor RLn constitute the measurement path.
In the Vc1p measurement period, voltage on which an influence of the insulation resistance RLp is reflected is measured. Thus, the switches S2 and S3 are turned on, the switches S1 and S4 are turned off, and the detection capacitor C1 is charged. That is, the high-voltage battery 300, the insulation resistor RLp, the ground, a resistor R5, the detection capacitor C1, and the resistor R2 constitute the measurement path.
It is known that, (RLp×RLn)/(RLp+RLn) can be derived based on (Vc1p+Vc1n)/V0 calculated from V0, Vc1n, and Vc1p obtained in these measurement periods. Thus, the control unit 420 of the ground fault detection apparatus 400 can figure out the combined resistance of the insulation resistances RLp and RLn by measuring V0, Vc1n, and Vc1p. If the combined resistance of the insulation resistances RLp and RLn is equal to or less than a predetermined determination reference level, then the control unit 420 determines that a ground fault is occurring and outputs a warning.
Meanwhile, in each of the measurement periods, when the detection capacitor C1 is fully charged, a voltage value of the high-voltage battery 300 is obtained in the V0 measurement period, and values obtained by dividing the high-voltage battery 300 by the insulation resistance RLp, RLn are just derived in the Vc1n measurement period and the Vc1p measurement period. The insulation resistance cannot be calculated by the above equation. For this reason, charge time in each of the measurement periods is set to be the time by which the detection capacitor C1 is charged for about 50%, for example.